In recent years, so-called “stop and start” control (abbreviated as “S & S” control hereinafter) of an engine has been adapted to reduce fuel consumption and emissions of automobiles. Under the S & S control, the engine is automatically stopped and restarted upon satisfaction of predetermined conditions. For example, the engine is stopped when an accelerator pedal is returned, and restarted when a brake pedal is returned.
However, when stopping or restarting the engine by the S & S control, an output torque of the engine would be varied significantly and hence a powertrain would be vibrated by such torque fluctuation. In order to reduce such vibrations, a clutch disposed between the engine and drive wheels is brought into disengagement to interrupt power transmission therebetween when stopping or restarting the engine by the S & S control. The clutch will be brought into engagement to allow power transmission between the engine and the drive wheels after the engine is restarted.
Basically, the above-mentioned clutch is actuated hydraulically by a mechanical pump driven by the engine. That is, the pump is also stopped when restarting the stopping engine and hence the hydraulic pressure is unavailable to actuate the clutch. Therefore, an optional accumulator is required to deliver hydraulic pressure to the clutch even when the engine is stopped by the S & S control.
One example of a vehicle drive device using such accumulator is described in JP-A-2010-151229. According to the teachings of JP-A-2010-151229, the accumulator is connected through a solenoid valve to a first oil passage of a hydraulic circuit connecting a shift valve to a forward clutch. A shut-off valve is also disposed on the first oil passage between the shift valve and a connection point with a second oil passage that is also connected to the accumulator. The solenoid valve opens the second oil passage just before activating the pump and closes the second oil passage just before inactivating the pump. The first oil passage is closed by the shut-off valve when delivering oil to the forward clutch from the accumulator.
JP-A-2010-151229 also describes a bypass passage extending parallel to the second passage to circumvent an orifice on the second oil passage, and a one-way valve disposed on the bypass passage to allow the oil to flow only toward the forward clutch.
According to the teachings of JP-A-2010-151229, the oil can be delivered promptly to the forward clutch from the accumulator through the bypass passage having the one-way valve when restarting the engine while using hydraulic pressure stored in the accumulator. By contrast, when the pump is driven by the engine, the oil is delivered from the pump to the accumulator through the second oil passage having the orifice. In this situation, since the oil is delivered through the orifice, the oil is stored into the accumulator slowly. That is, hydraulic pressure generated by the pump can be delivered promptly to the forward clutch when the engine is restarted and the pump is thereby activated. For this reason, the accumulator can be downsized.
According to the teachings of JP-A-2010-151229, however, the hydraulic pressure delivered to the forward clutch may not be returned to the accumulator smoothly. For example, given that a shift position of a transmission is shifted from a drive (D) position to a neutral position, the forward clutch has to be promptly brought into disengagement. However, the drive position would not be shifted smoothly from the D position to the N position if it takes too long to depressurize the forward clutch.
Such disadvantage may be solved by arranging a one-way clutch on the bypass passage taught by JP-A-2010-151229 to allow the oil to flow from the forward clutch only toward the accumulator. In this case, the forward clutch is allowed to be promptly brought into disengagement by returning the oil smoothly from the forward clutch to the accumulator. However, by contrast, it may take time to deliver the oil from the accumulator to the forward clutch to bring the forward clutch into engagement. Consequently, an engagement response of the forward clutch would be slowed when restarting the engine.
Alternatively, the time period to pressurize or depressurize the forward clutch may also be reduced by increasing the diameter of the orifice formed on the oil passage to the forward clutch. However, if the orifice is diametrically expanded to improve hydraulic response, vibrations and pulses of the oil caused by the solenoid valve would not be suppressed by the orifice and would propagate to the forward clutch. Consequently, controllability of the forward clutch would be degraded.
With the conventional hydraulic control system taught in the art, there still remains a need for improving hydraulic response to actuate the clutch device quickly and smoothly when automatically stopping and starting the engine.